Along with the digitization of information, it has become easier to share and use information over a broad range. Especially, needs for reusing paper-based documents as digital data (digital documents) are increasing. As a method of generating a digital document from a paper-based document (original), a method of reading a document image to search for an original digital document has been proposed (Japanese Patent No. 3,017,851).
As an example that improves the search efficiency in the above proposal, a system that reads a document image, then vector-converts objects of the image for respective blocks, and searches for a digital document based on layout information of the document image and vector information for respective blocks has been proposed.
Furthermore, when a document image is vector-converted and is reused as digital data, the necessity for falsification prevention and security protection is increasing. For example, Japanese Patent Laid-Open No. 2004-164538 describes a method of determining whether or not a document, the image of which is read, is a confidential document. That is, with this technique, the copy-forgery-prohibit-pattern of a confidential document is registered in advance, and it is checked if the copy-forgery-prohibit-pattern of the read document image matches the registered copy-forgery-prohibit-pattern, thus checking whether or not the read image is that of a confidential document.
Also, a proposal that changes security levels for respective documents is available. However, upon searching for a digital document of a document image, if information indicating a security level “vector conversion of objects is NG” is embedded in the document of interest, information of objects to be used in the search is deficient, thus disturbing satisfactory search processing. Or an enormously long search time is often required.